0022-4766/07/48 Supplement-0111 © 2007 Springer Science+Business Media, Inc. S111

Journal of Structural Chemistry, Vol. 48, Supplement, pp. S111-S124, 2007

Original Russian Text Copyright © 2007 by I. V. Yudanov

MECHANISM OF OLEFIN EPOXIDATION WITH

TRANSITION METAL PEROXO COMPLEXES:

DFT STUDY

I. V. Yudanov UDC 539.194+541.128.13

Using density functional calculations over the last decade led to considerable progress in understanding the

mechanism of olefin epoxidation with Ti, V, Mo, W, and Re peroxo complexes. According to calculations,

the reaction occurs by direct electrophilic transfer of one of the atoms of the peroxo group to the olefin. The

alternative stepwise mechanism, which has been discussed for a long time and suggested the formation of a

metallocyclic intermediate, is characterized by higher activation barriers than direct transfer. The

electrophilic character of the direct transfer of oxygen was interpreted at the level of molecular orbital

analysis as interaction between the HOMO of the olefin (C–C) and the LUMO of the peroxo group

*(O–O). The factors determining the activity of various metal complexes in epoxidation were examined in

relation to the ligand environment and the structure of the peroxo group.

Keywords: Ti, V, Cr, Mo, W, and Re peroxo complexes; epoxidation of olefins, oxygen transfer, activation

barriers, ab initio calculations.

INTRODUCTION

Since the start of the 1970s, transition metal peroxo compounds of Group IV-VII elements at higher degrees of

oxidation of the periodic table (TiIV, VV, MoVI, WVI, and ReVII) have found wide use in oxygen transfer processes [1-5].

Epoxidations of unsaturated hydrocarbons are the most important processes because the epoxides formed by oxygen transfer

to the C=C double bond are the key intermediates in the polymer synthesis and production of pharmaceuticals. In 1970,

Mimoun et al. synthesized bisperoxo complexes MoO(O2)2 hmpt (hmpt = hexamethylphosphorus triamide),

stoichiometrically oxidizing olefins into epoxides at room temperature [6]. This system was a prototype for a family of

molybdenum and tungsten bisperoxo complexes MO(O2)2 L (M = Mo, W) with various basic ligands L, which are active in

epoxidation [7]. Another important step was made when Katsuki and Sharpless discovered asymmetric epoxidation of

unsaturated alcohols with tert-butyl hydroperoxide tBuOOH, catalyzed by TiIV complexes [8] (for his works on asymmetric

synthesis Sharpless was awarded the Nobel prize in 2001 [9]). Further studies showed that the key role in the

Katsuki Sharpless process was played by the intermediate with the alkylperoxo group coordinated to the titanium ion of

TiOOtBu [10]. Today great attention is paid to the development of new catalytic systems based on transition metal

compounds using hydrogen peroxide H2O2 as an environmentally safe oxidizer [11]. In catalytic systems based on, e.g., Ti

silicates [12] and methyltrioxorhenium CH3ReO3 (MTO) [13, 14], the role of active intermediates in epoxidation is played by

TiIV and ReVII peroxo complexes, respectively, formed by interaction of H2O2 with the catalyst.

G. K. Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk; ilya@catalysis.ru. Translated from Zhurnal Strukturnoi Khimii, Vol. 48, Supplement, pp. S117-S131, 2007. Original article submitted March 13, 2007.

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Although the mechanism of epoxidation with transition metal peroxo complexes is very important in practice, many

aspects of the mechanism of epoxidation are debatable. In this work we attempted to describe recent progress in

understanding the epoxidation mechanisms owing to wide use of theoretical calculations (see also [15-17]) by modern

quantum-chemical methods with high predictive power, primarily, by the density functional method.

EVOLUTION OF THE CONCEPTS OF EPOXIDATION WITH PEROXO COMPLEXES

Mimoun and his colleagues, who discovered epoxidation with bisperoxo complexes MoO(O2)2 hmpt (1, Fig. 1),

investigated the reaction kinetics and suggested a multistage mechanism [6], according to which the first stage is olefin

coordination to metallocenter (2) with further incorporation in one of the bonds between the metallocenter and the peroxo

group, leading to the formation of five-membered metallocycle 3, including a metal center, two carbon atoms, and the oxygen

atoms of the peroxo group. It is assumed that the epoxide molecule is released from the metallocycle at the final stage (4).

However, metallocyclic intermediates similar to 3 are known only for complexes of the elements from the end of the

transition row of the periodic table, e.g., Pd and Pt [18-20], and were not found in reactions involving transition metal

peroxides with a d 0 configuration on the metal center. Mimoun extended the stepwise mechanism to the epoxidation reactions

of olefins with alkyl hydroperoxides catalyzed by TiIV, VV, and MoVI [21].

A concerted mechanism was suggested as an alternative to the stepwise mechanism of Mimoun. Sharpless et al. [22]

assumed that olefin incorporation in the coordination sphere is not an essential condition, and epoxidation with the

MoO(O2)2 hmpt complex occurs by a direct attack at the double bond of the olefin at one of the oxygen centers of the peroxo

group via the transition state, which has a structure with a spiro conformation (5, Fig. 1). Despite further wide experimental

studies of the reaction mechanism and the discovery of new systems in which epoxidation was effected by transition metal

peroxo compounds, the contradiction between the two mechanisms was not solved until recently, although most researchers

favored Sharpless’ concerted mechanism [23-25]. The lacking arguments in favor of this mechanism were obtained recently,

by comparing the alternative reaction mechanisms by quantum-chemical calculations of the corresponding reaction routes,

inclusing the transition states.

In addition to the debatable problem of the general mechanism of oxygen transfer from the peroxo complex to the

olefin, there are other open questions, namely, the role of basic ligands in control over the activity of peroxo complexes, the

role of the coordinating metal center in activation/deactivation of the peroxo group, relationship between the activity and

Fig. 1. Stepwise mechanism of epoxidation via the formation of metallocycle 3 suggested by Mimoun and concerted mechanism of the direct oxygen transfer via transition state 5 suggested by Sharpless. In the paper of Mimoun et al., L = hmpt. The transition states for the formation of metallocycle 3 and final products 4 in Mimoun’s mechanism are not shown.

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stability of the mono and bisperoxo complexes, reasons for activity of complexes with a symmetric 2–O2 peroxo group in

some systems and of alkyl and hydroperoxo intermediates in others, and the mechanism of stereoselectivity control of

epoxidation.

SEMIEMPIRICAL AND EARLY AB INITIO CALCULATIONS OF PEROXO COMPLEXES

A number of theoretical works appeared over the 1980-1990s, in which the interaction of peroxide with a d 0 metal

center was studied to clarify the mechanism of oxygen transfer from the transition metal peroxo complex to alkene [1, 26-31]

or sulfide [32]. In view of the limitations of the approaches used (semiempirical methods and the Hartree Fock method), the

works were confined to molecular orbital analysis of the ground state of the peroxo complex. Nevertheless, some authors

attempted to directly consider the separate stages of epoxidation. Thus, the earliest theoretical study [26, 27] of the

mechanism of epoxidation using extended Hückel’s method (EHM) considered decomposition of the five-membered

metallocyclic intermediate formed, according to Mimoun’s assumption [18], during the interaction of the molybdenum

peroxo complex with alkene. In another EHM study, an original mechanism was suggested according to which ethylene (as

model alkene) is at first coordinated to the metal center of the MoO(O2)2 complex and then “glides” to one of the oxygen

centers of the peroxo group, forming with it a three-membered transition state [29]. The electrophilic properties of the six-

and seven-coordinate molybdenum peroxo complexes were compared using the NDDO approximation by calculating the

electron affinity of the complex [33]. The electron affinity of the six-coordinate complex proved higher than that of the

seven-coordinate complex, which is indirect evidence in favor of the Sharpless mechanism, which implies a direct

electrophilic attack of the peroxo group at the double bond of the olefin. Among semiempirical approaches one can also

mention parametrization of the AM1/d method for molybdenum [34], which permits one to describe the structure and

energies of the ground states of the MoVI peroxo complexes.

DENSITY FUNCTIONAL THEORY CALCULATIONS

Great progress in the mechanistic studies of epoxidation was achieved during recent 10 to 15 years in view of the

development of density functional theory (DFT). Using this method permits one to perform calculations for large systems

such as transition metal complexes with complex organic ligands. Using the most effective exchange correlation functionals

such as B3LYP [35, 36] makes it possible to obtain data with high predictive power for the epoxidation energies and the

activation barriers of oxygen transfer. Some of the DFT calculations are aimed at investigating various structural

characteristics of peroxo complexes [37-40]. Other authors considered directly the mechanism of epoxidation; they sought the

transition states and calculated the activation barriers for different stages corresponding to a definite reaction pathway. Thus,

epoxidation of olefins with Ti [41-50], V [51], Cr/Mo/W [52-57], and Re peroxo compounds [58-60] was studied.

Mechanistic studies have also been recently performed for other oxygen transfer processes, in particular, for sulfide

oxidation into sulfoxides with Mo [61, 62] and V [63, 64] peroxo complexes. Oxidation of halides into V peroxo complexes

was also studied in view of the interest of researchers in the mechanism of the enzymatic action of vanadium haloperoxidase

[65, 66].

Many studies of the epoxidation mechanism were performed with the use of ethylene as the simplest model olefin in

the approximation that neglected the medium effect. The solvent effect on the activation barriers of epoxidation was initially

investigated on simple examples of organic peroxo compounds, namely, dioxirane and acetyl percarboxylic acid [67]. The

resulting relation between the dielectric constant of the solvent and variation of the activation barrier relative to the gas-phase

reaction was successfully used for evaluating the solvent effect on the activity of the CH3Re(O)(O2)2 L peroxo complex

(L = H2O, pyridine, pyrazole) [67]. In recent works, the solvation corrections were included using the polarized continuum

model directly in calculations of peroxo complexes and the corresponding transition states during the oxygen transfer [62, 66,

68, 69].

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Below we consider in detail some aspects of the activity of transition metal peroxo compounds in epoxidation of

unsaturated hydrocarbons, investigated by the DFT method in recent years.

MECHANISM OF OXYGEN TRANSFER

Quantum-chemical calculations using the B3LYP functional allowed us to reject the Mimoun mechanism of

epoxidation and choose the Sharpless mechanism. The two mechanisms were compared using as an example calculations [52,

54] for MoO(O2)2 L complexes (1) with various basic ligands L, including OPH3, which was used by Mimoun as a hmpa

model ligand in his original work [6]. Regarding Mimoun’s stepwise mechanism it was found that olefin coordination (with

ethylene used as a model olefin) was thermodynamically unfavorable (the endothermal effect was 5 kcal/mol); and there

was no transition state leading from intermediate 2 (Fig. 1) with coordinated ethylene to the five-membered metallocycle 3

[54]. According to the calculations, ethylene incorporation into the bond between molybdenum and the peroxo oxygen and

the formation of a five-membered ring occur directly from the outer coordination sphere of the complex, without preliminary

coordination to the metal center. However, the transition state of ethylene incorporation is characterized by a high activation

barrier, 25 kcal/mol [52, 54], and intermediate 3 containing the five-membered ring is higher in energy than the starting

peroxo complex and the nonbonded ethylene molecule (1). Moreover, it appeared that even if a metallocycle formed despite

the high activation barrier, the decomposition product of this ring could only be aldehyde, but not epoxide, because there is

no transition state that leads to the epoxide [54]. It should be noted that the aldehyde is thermodynamically more stable than

epoxide, and the calculated – bond (1.52 Å) in the five-membered ring is much closer to the corresponding characteristic

in the aldehyde molecule (1.51 Å) than in epoxide (1.47 Å).

According to calculations, the alternative concerted mechanism suggested by Sharpless is characterized by a

transition state in the form of a three-membered ring (5, Fig. 1), whose product is epoxide [52, 54]. The transition state has a

spiral structure, in which the axis of the =C bond is orthogonal to the plane of the peroxo group interacting with it. The

corresponding activation barrier is much lower than the activation barrier of the above-discussed incorporation of ethylene,

leading to the formation of a metallocycle [52, 54]. Similar results, demonstrating that the concerted mechanism is favorable

in view of lower activation energies, were also obtained for [53] monoperoxo complexes; for Cr and W [52] mono and

bisperoxo complexes, which are the structural analogs of the corresponding Mo complexes; Re mono and bisperoxo

complexes [58]; Ti peroxo complexes [49]; and V complex, [VO(O2)2(imidazole)]– [51]. Thus, all of the peroxo complexes

under study with the d 0 electron configuration on the metal center undergo epoxidation according to the single concerted

mechanism, which also takes place in the epoxidation with organic peroxo compounds such as dioxiranes and peracids [70-

75]. According to calculations, oxidation of sulfides with vanadium peroxo complexes also occurs by direct oxygen transfer

[63].

ELECTROPHILIC CHARACTER OF OXYGEN TRANSFER

While the mechanism of epoxidation is common to all peroxo complexes, the latter differ in activity in this reaction,

which depends on the surrounding ligands, the nature of the metal center, and some other factors. To explain the different

reactivities of the peroxo complexes, it is useful to perform molecular orbital analysis. This analysis was performed in detail,

for example, for a series of structurally related rhenium complexes, CH3Re(O2)2O L (see structure 11 below) with different

Lewis bases as ligands L [60]. The key point of molecular orbital analysis of the interaction between the peroxo complex and

the olefin molecule is to establish a relationship between the activation barrier of oxygen transfer and the energies of the

frontier orbitals of the system, namely, the highest occupied molecular orbital (HOMO) (C–C) of olefin and the lowest

unoccupied molecular orbital (LUMO) *(O–O) of the peroxo group [60].

According to the concept about the electrophilic character of the peroxo group attack at the double bond of the olefin

[16, 17], the electron density is partially transferred from the (C–C) of the olefin to the *(O–O) antibonding orbital of the

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Fig. 2. Orbital interaction between the peroxo group coordinated to the metal ion and the olefin molecule. The || and symbols denote the orbitals of the peroxo group lying in the MO2 plane and orthogonal to this plane, respectively.

peroxo group (Fig. 2); occupation of the latter leads to the O–O bond cleavage (according to the calculated atomic

populations, the charge transfer to the peroxo complex in the transition state of the interaction with ethylene is ~0.2 e [49, 58,

60]). Indeed, as shown for the CH3Re(O2)2O complex, the activation barrier of oxygen transfer decreases as the electron

density on the double bond of the olefin increases as a result of sequential introduction of the inductive methyl substituents

into the ethylene molecule [60]. The dependence of the activation barrier on the HOMO energy of the olefin is virtually linear

[60]. In a similar way, the electrophilicity of the peroxo group and, accordingly, the position of the *(O–O) level may be

“adjusted” by introducing various basic ligands in the coordination sphere of the complex. Thus, in the series of

CH3Re(O2)2O L complexes, the activation barrier increased with the *(O–O) energy level [60]; a similar result was obtained

for Ti [49] and Cr/Mo/W complexes [52]. It is difficult to unambiguously choose a molecular orbital with the dominant

*(O–O) contribution because of an admixture of the vacant d orbitals of the metal center. Thus, it seems that the dependence

of the activation barrier on the energy of the *(O–O) level is purely qualitative, while the dependence of the barrier on the

clearly defined HOMOs of the olefin is predictive to a certain extent. Extrapolation of the dependence of the activation

barrier on the HOMO of olefin, obtained for methyl-substituted derivatives of ethylene, made it possible to predict the

activation barrier of epoxidation of 4-methoxystyrene in good agreement with the experimental data [16]. Naturally,

calculation of orbital energies demands much lower computing time that the search for the transition state for a substrate as

large as 4-methoxystyrene. The dependences obtained were successfully applied to the reactivity analysis of organic

peroxides [67, 76, 77].

The theoretical concepts about the electrophilic character of olefin epoxidation with transition metal peroxo

compounds are consistent with experimental data. Thus, it is known that epoxidation of alkyl-substituted olefins is faster for

those olefins that have increased electron density on the C=C bond. Sulfoxidation of thianthrene-5-oxide was used as an

experimental test for electrophilicity of oxygen transfer [78]. Since the thianthrene-5-oxide molecule (6) contains both sulfide

and sulfoxide functional groups, the oxygen transfer predominantly to the sulfide center (6 7) means that the reaction is

electrophilic with respect to the peroxo group. This test demonstrated the electrophilic character of the peroxo group in V,

Mo, and W complexes [79, 80]. In recent years, several theoretical works considered the oxidation of vanadium sulfides [63,

64] and molybdenum peroxo complexes [61, 62].

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BASIC LIGAND EFFECTS

The basic ligand effects on the activity of the peroxo group is an important aspect of the mechanism of olefin

epoxidation with transition metal peroxo compounds. For Mo and W bisperoxo complexes (MO(O2)2L, where M = Mo, W; L

is a basic ligand), the deactivating effect of the highly coordinating solvents is considered in conjunction with the general

reaction mechanism [6, 18, 22, 23, 25]. According to Mimoun’s mechanism [6, 18], the solvent molecule occupies a

coordination vacancy at the metal ion, thus blocking olefin precoordination, which is the first stage of the formation of the

metallocyclic intermediate [81]. In the (Sharpless’) direct attack mechanism, the solvent effect induces a shift of the electron

density from the extra ligand (solvent molecule) via the metal center to the peroxo group, which decreases the electrophilicity

of the latter [23, 25, 82].

The choice of the basic ligand is also of prime importance for the MTO/H2O2 catalytic system, in which ReVII mono

(10) and bisperoxo complexes (11) are believed to be the epoxidizing intermediates [14]. It was shown that using the adducts

of Lewis bases with MTO (9) considerably reduced the formation of undesirable diols due to the reduced Lewis acidity of the

system [83]. However, while decreasing the selectivity of oxidation, the use of bases decreased the epoxidation rate, as in the

case of Mo and W peroxo complexes. It appeared that using a two-phase system (aqueous phase/organic phase) with a

pyridine excess as a Lewis base hindered diol formation and increased the reaction rate compared to the starting catalyst

based on [84-86]. It was shown that using 3-cyanopyridine and especially pyrazole as Lewis bases raised the efficiency

of the process relative to pyridine [87, 88], while pyridine N-oxide proved to be less effective [14, 89, 90].

According to calculations for transition metal peroxo complexes with a crowded coordination sphere, the activation

barriers of epoxidation increase considerably compared with those for similar complexes having vacancies not filled with

basic ligands. A typical example is an increase in the calculated activation barrier of ethylene epoxidation from 14 kcal/mol

for MoO(O2)2 NH3 (12) to 20 kcal/mol for MoO(O2)2 (NH3)2 (13) [52]. The addition of the second basic ligand is

thermodynamically favorable; in this case, the binding energy between the NH3 model base and the complex is 15 kcal/mol

[52].

The activation barrier increases even to a greater extent, from 19 kcal/mol for (HO)2Ti(O2) NH3 to 27 kcal/mol for

(HO)2Ti(O2) (NH3)2 (see structure 23 below) when the coordination sphere of model Ti complexes is saturated [49]. These

results agree with the experimental data on reduced activity of MoO(O2)2 hmpt type molybdenum complexes in highly

coordinating solvents and inactivity of the known titanium complexes (TPP)Ti(O2) [91] (TPP = tetraphenylporphyrin) and

Ti(O2)(pic)2 hmpt [92] (pic = the deprotonated picolinic acid anion) in epoxidation.

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Since it is difficult to create conditions for the existence of peroxo complexes with an unoccupied coordination

sphere, it is important to choose bases that should form a stable bond with the complex, on the one hand, and be moderately

electron inductive, on the other, in order to reduce the electrophilic properties of the peroxo complex and accordingly

increase the activation barrier of epoxidation. An interesting study was performed for bisperoxo complexes CH3ReO(O2)2 L

(11), for which calculations were carried out with the following set of basic ligands: L = H2O, NH3, NMe3, pyridine (14),

pyridine-N-oxide (15), and pyrazole (16) [60].

It appeared that the most stable adducts with peroxo complex 11 were formed by NH3 and pyrazole (the binding

energy was ~20 kcal/mol), while the adducts formed by pyridine and pyridine-N-oxide ( 18 kcal/mol) were slightly less

stable. However, the calculated activation barrier to epoxidation of ethylene with a peroxo complex was 15 kcal/mol for

pyrazole as a ligand, 16 kcal/mol for pyridine, 18 kcal/mol for pyridine-N-oxide, and 20 kcal/mol for NH3 [60]. Thus, to

ensure the high rate of epoxidation, it is preferable to use pyrazole as a base; due to the stability of the bond, it is not removed

by other bases from the complex and provides a lower-energy transition state of oxygen transfer. Indeed, as was found in

experiment, during epoxidation of styrene in an MTO/H2O2 catalytic system, the highest activity is achieved when pyrazole is

used as a Lewis base, and pyridine is the next effective base [85-88].

As noted above, the inhibiting effect of the basic ligands on the activity of transition metal peroxo complexes in the

epoxidation reaction was initially explained by their blocking of olefin precoordination to the metal center [6, 18]. However,

as shown by calculations, the reaction occurs by the concerted mechanism, which does not require olefin precoordination, and

the effect of the basic ligands is probably reduced to modification of the electronic structure of the peroxo complex. In the

ionic approximation, the interaction of the peroxo group with the metal center can be represented as ionic interaction between

the 22O peroxide ion and the metal ion with a d 0 configuration (e.g., TiVI, MoVI, ReVII). Both the (O–O) bonding and

*(O–O) antibonding orbitals of the peroxo group are filled completely, and any increase in the electron density on the

peroxo group shold lead to the occupation of the *(O–O) antibonding orbital and, accordingly, to O–O bond cleavage. The

ionic model, however, is a simplification, while in fact the metal peroxide bond has a significant covalent contribution, and

some part of electron density is redistributed between the peroxo group and the metal center (see the molecular orbitals in

Fig. 2). Population analysis generally gives the charge of the peroxo group that is not compensated by the cores of the oxygen

centers (close to 1e). Thus, the metal center acts as a Lewis acid, withdrawing the electron density from the peroxo group

[93]. The base that is coordinated to the metal center acts as an electron density donor with respect to the metal center. When

the electron density on the metal center increases as a result of coordination of an extra basic ligand, for example, pyridine,

the electron density is slightly shifted from the metal center to the peroxo group. The increase in the electron density on the

peroxo group, detected by population analysis, is relatively low, generally 0.02-0.04 e [49, 52]. The core levels of the O1s

oxygen centers are shifted 0.7-1.2 eV toward higher energies [49, 52]. The vacant *(O–O) is also appreciably sifted upward,

which decreases the electrophilicity of the peroxo group during the olefin attack and shows itself as increased activation

barrier of epoxidation [49, 52, 60]. It is also important that the donor orbitals of the basic ligands lie closer in energy to the

low-lying (O–O) bonding orbitals than to the *(O–O) antibonding orbitals of the peroxo group, and, accordingly, are

mixed to a greater extent with the (O–O) bonding orbitals of the peroxo group in the molecular orbital structure of the

complex (see, e.g., orbital analysis for the case of coordination of NH3 molecules to the titanium peroxo complex [49]). Thus,

one can argue that an increase in the electron density on the peroxo group affects the (O–O) bonding levels to a greater

extent, which leads to stabilization of the O–O bond [49].

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ACTIVITY OF MONO- AND BISPEROXO COMPLEXES

The transition elements of Subgroups V (vanadium), VI (Cr, Mo, W), and VII (Re) are capable of forming not only

monoperoxo complexes with one peroxo group, but also bisperoxo complexes with two peroxo groups [5, 7, 13]. Molybdenum

bisperoxo complexes were first used by Mimoun for epoxidation of olefins [6]. These stoichiometric oxidants served as a

convenient model system for experimental studies of the reaction mechanism. Modern approaches to chemical synthesis,

however, are aimed at developing catalytic systems in which intermediates of different types and different levels of activity

can be generated. Thus, the nature of active intermediates in the MTO/H2O2 catalytic system was discussed [94-96]; in this

system, the CH3ReO(O2)2(H2O) rhenium bisperoxo complex, formed by the interaction of MTO with H2O2 and stabilized by

the aqua ligand, was isolated, and its structure was characterized [97]. However, monoperoxo intermediates 10 are also

considered as intermediates of epoxidation [98], leading to synthesis of bisperoxo complexes.

Calculations on the interaction of trioxomolybdenum MoO3 with H2O2 in the presence of H2O and OPH3 as bases

showed that sequential perhydrolysis of two oxo ligands was exothermal, and the most stable product was the

MoO(O2)2 OPH3 H2O bisperoxo complex; perhydrolysis of the third oxo group formed the Mo(O2)3 triperoxo complex and

was thermodynamically unfavorable [55]. Calculations of activation barriers to ethylene epoxidation indicated that the Mo

bisperoxo complexes were more active than the monoperoxo complexes in this reaction [52, 53]. The most consistent

analysis of the activity of mono- and bisperoxo complexes is given in [62]; in the MoO(O2)2 H2O NH3 bisperoxo complex,

one of the peroxo groups was substituted by different anion substituents without changing the rest of the environment. For all

monoperoxo complexes obtained in this way (17-20), the activation barrier to ethylene epoxidation was 2-7 kcal/mol higher

than for the starting bisperoxo complex, which indicated that the latter was more active [62].

Calculations for Re complexes demonstrated that the monoperoxo- and bisperoxo intermediates, H3ReO2(O2) H2O

and H3ReO(O2)2 H2O (structures 10 and 11, L = H2O), were close in stability (the conversion of the monoperoxo complex

into the bisperoxo complex gave an exothermal effect of ~3 kcal/mol) and had almost the same activation barriers to ethylene

epoxidation [58]. It should be borne in mind that the calculated values can be slightly shifted if medium effects are taken into

account, but in the approximation used, epoxidation possibly involves both types of rhenium mono- and bisperoxo

complexes.

METAL CENTER EFFECT ON THE ACTIVITY OF THE PEROXO COMPLEX

The Subgroup VI transition elements (Cr, Mo, and W) at the highest degree of oxidation form isostructural

bisperoxo complexes MO(O2)2 Ln [7], which show different activities in olefin epoxidation. Thus, MoO(O2)2 hmpt was the

first peroxo compound capable of stoichiometrically epoxidizing alkenes in nonpolar solvents, and was further used as an

oxidant in a number of studies [7]. Its structural analog, WO(O2)2 hmpt, was shown to be more active in epoxidation [23],

while the structurally related CrVI peroxo complexes were absolutely inert in the oxygen transfer reactions [7]. Since a

number of MO(O2)2L1L2 complexes (M = Cr, Mo, W) with different combinations of the basic L1 and L2 ligands were

characterized experimentally [7], these systems are convenient objects for theoretical analysis of the role of the metal center

in activating or deactivating the peroxo group coordinated to it [15, 52, 57].

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According to calculations for a series of model monoperoxo complexes MO2(O2) (NH3)n (21, 22) and bisperoxo

complexes MO(O2)2 (NH3)n (M = Cr, Mo, W; n = 1, 2; see, e.g., structures 12 and 13), the activation barriers of ethylene

epoxidation decrease in a sequence Cr Mo W [52]. Thus, for MO(O2)2 NH3, the calculated activation barriers are

19 kcal/mol, 14 kcal/mol, and 11 kcal/mol for M = Cr, Mo, and W, respectively. As for structural characteristics, the O–O

bond length in the peroxo group increased from 1.40 Å in the Cr complex to 1.45 Å and 1.48 Å in the Mo and W complexes,

respectively [52]. Thus, the O–O bond length, which reflects the bond stability, correlates with the activation barrier; the

shorter the bond, the higher the barrier. At the same time, the Cr–O bonds between the Cr ion and the peroxo oxygen sites

(1.80-1.85 Å) are shorter than the corresponding bonds in the Mo and W complexes (1.94-1.97 Å); this is explained by the

smaller ionic radius of Cr, which is an element from the first row of transition metals. The minor difference between the ionic

radii of the elements from the second (Mo) and third (W) rows of transition elements is explained by “lanthanide

compression” of the electron shells in the third transition row due to the relativistic effects (for Subgroup VI peroxo

complexes, the relativistic effects are considered in detail in [57]). Population analysis indicates that polarization of the W–O

and Mo–O bonds increased compared with that of Cr–O [52]. Thus, in Cr peroxo complexes, the electron density is shifted

from the *(O–O) highest occupied levels to the formally vacant Cr d levels to a greater extent than in the isostructural Mo

and W complexes; this stabilizes the bond in the peroxo group and, accordingly, leads to its deactivation in oxygen transfer

[52, 15]. At the same time, the above-discussed correlation between the activation barrier of oxygen transfer and the energy

of the vacant *(O–O) antibonding orbital of the peroxo group persists; the higher activation barrier in the Cr complexes

corresponds to the higher position of the *(O–O) compared with that in the Mo and W complexes [52, 15].

Thus, among Subgroup VI transition metal peroxo complexes, the W complexes show the highest activity in

epoxidation. Even if the coordination sphere of the W complexes is saturated with basic ligands, the activation barrier of

oxygen transfer remains moderate; for example, the calculated activation barrier of ethylene epoxidation is 17 kcal/mol for

WO(O2)2 (NH3)2 and 20 kcal/mol for MoO(O2)2 (NH3)2 [52]. It is interesting that among complexes of the third-transition

row element Re, the isoelectronic analog of the WO(O2)2 (NH3)2 complex is the bisperoxo complex CH3ReO(O2)2 NH3, for

which a similar calculation gave an activation barrier of ~20 kcal/mol [60]. Thus, along with the tendency toward increasing

activity of peroxo complexes in passing from light to heavier elements within the subgroup, there is a tendency toward

lowering of the activity of the complexes of the elements lying in the middle of the transition row. Indeed, when the atomic

number increases in the middle of the transition row, the stability of the d 0 ions decreases, which, in turn, leads to a decrease

in the polarizability of the bond between the metal center and the peroxo group and stabilization of the bond in the peroxo

group. Thus, a comparison of the characteristics of the metal peroxo groups in complexes of the first-transition row elements

TiIV [49] and CrVI [52] pointed to higher polarization of the Ti–O bond compared with that of the Cr–O bond and to longer

O–O bonds in the Ti complexes (1.46-1.47 Å).

ALKYL- AND HYDROPEROXO INTERMEDIATES

Many Ti, V, Cr/Mo/W, and Re peroxo complexes with symmetric 2 coordination of the peroxo group to the metal

center were characterized experimentally. However, few complexes with an alkyl peroxo group were isolated and

characterized by XRD analysis, namely, (di(picolinato) VO(OOtBu)(H2O) [99] and [(( 2-tert-butylperoxo) titanatrane)2 3

dichloromethane] [100]. The structure of some other alkylperoxo complexes was suggested based on NMR data analysis

[101]. At the same time, alkyl- and hydroperoxo intermediates presumably play the key role and are oxygen donors in

important catalytic processes such as Sharpless’ epoxidation [10] and epoxidation on titanosilicalites [102, 103], although

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previous reports on epoxidation on titanosilicalites also assumed that intermediates with a symmetric Ti(O2) peroxo group

played an active role [104, 105]. Recent experimental [106, 107] and calculated data [107] indicated that in titanosilicalite

systems in the presence of hydrogen peroxide and water, there was an equilibrium between hydroperoxo- and 2-peroxo

intermediates. However, all known stable peroxo complexes with a symmetric Ti( 2–O2) peroxo group are not involved in

epoxidation [91, 92, 108]. Nevertheless, it was shown that after its transformation into the (TPP)Ti(OH)(OOR) alkylperoxo

complex, (TPP)Ti(O2) (TPP = tetraphenylporphyrin) epoxidizes cyclohexane [91]. In studies of oxidation with hydrogen

peroxide on catalysts whose active component is isolated TiIV site, the similarity between the properties (structure and

activity) of heterogeneous and homogeneous catalytic systems is also of interest [109].

A comparative theoretical study of the factors responsible for the different activities of TiIV complexes bearing the

=Ti( 2–O2) (e.g., 23) and TiOOR (24) groups (R = H, CH3) was performed in [49]. A number of computational works

dealed with epoxidation of olefins with TiOOH hydroperoxo intermediates [41-44, 47, 48, 68], electron transfer from the

H2O2 molecule coordinated to the Ti metal center [45] and from the Ti–O–O–Si peroxo group [50]. Wells et al. compared the

three mechanisms of propylene epoxidation [110] and studied epoxidation with the TiOOH hydroperoxo group localized on

the defective structure of the lattice near the silicon vacancy.

Because of the proton, the TiOOH group is rather asymmetric; the Ti–O bond length (see structure 24) in the

model complexes (~1.9 Å) is close to the corresponding characteristic in the symmetric Ti( 2–O2) peroxo group, while the

Ti–O bond length exceeds 2.2 Å [49]. The structure of the transition state of ethylene epoxidation with hydroperoxo

intermediates is very similar to the structure of the above-considered transition states of oxygen transfer from the peroxo

complexes with symmetric coordination of the 2–O2 peroxo group. The activation barrier of the transfer of the O oxygen

site is much lower because the attack at the O site is accompanied by a proton transfer (the O site, which remains in the

complex, generally acts as an acceptor) [49]. In contrast to the symmetric Ti( 2–O2) peroxo group, the TiOOH hydroperoxo

group is almost insensitive to the deactivating effect of the basic ligands. Thus, for the (HO)3TiOOH(NH3) model complex

with a saturated coordination sphere, the activation barrier of ethylene epoxidation was calculated to be rather low,

12 kcal/mol, while the corresponding characteristic of the (HO)2Ti( 2–O2)(NH3)2 complex was 26 kcal/mol (in its

surroundings, the latter complex can serve as a model of (TPP)Ti(O2), which is inactive in epoxidation) [49]. The low

sensitivity of TiOOH hydroperoxo intermediates to the effects of the basic ligands is primarily associated with the interaction

between the metal center and the peroxo group, which is weaker than Ti( 2–O2). Indeed, only the O site interacts effectively

with the Ti ion, but here again the Ti–O bond length, ~1.9 Å, is slightly larger than the Ti–O bond lengths in the symmetric

Ti( 2–O2) peroxo group, 1.82-1.86 Å [49]. Proton substitution in TiOOH with a methyl substituent leads to a 3 kcal/mol

increase in the activation barrier of ethylene epoxidation, which is explained by the inductive effect of the methyl substituent,

decreasing the electrophilicity of the peroxo group and increasing the energy of the *(O–O) level [49].

Recently, it was found that the MoO(O2)2(L–L) seven-coordinate complexes (the bidentate ligand L–L =

pyrazolepyridine) are capable of catalyzing the epoxidation reaction if t-BuOOH is used as an oxidant [111-113]. It was

shown that none of the 2–O2 peroxo groups is involved in the oxygen transfer, and the active center was assumed to be the

tert-butylperoxo group coordinated to Mo, MoOOt-Bu [112, 113]. Calculations of MoO(O2)(OOH)(OOCH3)(NH3)2 type

model complexes (25) showed that they form exothermally during the interaction of the starting MoO(O2)2(NH3)2 bisperoxo

complex [114], but the hydrogen bonds in the complex obviously produce a considerable stabilizing effect. Another

theoretical work studied hypothetical MoVI hydroperoxo intermediates 26 along with ethylene epoxidation with similar ReVII

structures 27 [115]. Deubel et al. showed that protonation of one of the peroxo groups in the MoO(O2)2(OPH3) bisperoxo

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complex led to a considerable decrease in the activation energy of ethylene epoxidation [116]. Recently, the mechanism of

formation of active center 28 by interaction of the MoO2X2L dioxo complex (X = Br, Cl) with methylhydroperoxide [117,

118] was considered. Calculations of energies and activation barriers of ethylene epoxidation showed that MoVI hydro- and

alkylperoxo intermediates could be active in oxidation of hydrocarbons [15, 115, 117]. For ReVII systems, this mechanism is

less probable [115].

STEREOSELECTIVITY OF THE OXYGEN TRANSFER

This work concentrates on the mechanism of oxygen transfer and the factors that govern the activity of the peroxo

group coordinated to the transition metal ion in this process. A number of works reported on theoretical studies of the

mechanism of stereoselectivity control over epoxidation of unsaturated hydrocarbons, started in the semiempirical work by

Jorgensen et al. [30] and continued by calculations at the DFT level [42, 69, 119]. Most theoretical works in this direction

focused on the mechanism of stereoselective epoxidation of unsaturated alcohols in the Katsuki Sharpless process catalyzed

by TiIV complexes [30, 42, 69]; Rösch et al. studied epoxidation of unsaturated alcohols with ReVII complexes in the

MTO/H2O2 system [119]. Major factors that govern stereoselectivity are as follows: 1) coordination of the hydroxyl group of

the substrate being oxidized (with the use of a lone electron pair) to the metal center; 2) formation of the M–OR metal–

alkoxide bond; 3) hydrogen bonding between the hydroxyl group of the unsaturated alcohol and one of the oxygen-containing

ligands of the peroxo complex [16, 119]. In the case of oxidation on TiIV complexes, the formation of metal alkoxide

alkylperoxo intermediates, in which both the substrate and oxidant are bonded to the same TiIV center is the dominant process

[42, 69]. At the same time, based on the calculations of various reaction routes of interaction between the CH3Re(O)(O2)2

H2O bisperoxo complex and propylene, it was concluded that oxygen transfer could preferably occur from the peroxo group

to the double bond of propylene via the intermediates and transition states in which propylene forms a hydrogen bond to the

peroxo oxygen atoms [119]; the mechanism that forms alkoxide intermediates is unfavorable for ReVII complexes, which

agrees with the experimental data of [120].

CONCLUSIONS

DFT calculations of different reaction routes permitted us to draw conclusions about the mechanism of olefin

epoxidation with transition metal peroxo complexes with the d 0 electron configuration. The reaction occurs by direct

electrophilic transfer of one of the peroxo oxygens to the olefin via a transition state with a spiral structure (Sharpless

mechanism). Activation and cleavage of the O–O bond in the transition state occur by interaction between the (C–C)

HOMO of the olefin and the *(O–O) LUMO of the peroxo group. The Mimoun’s mechanism, which suggests that the olefin

is incorporated into one of the bonds between the metal center and the peroxo group, is characterized by a higher activation

barrier than the direct transfer. Moreover, decomposition of the five-membered metallocyclic intermediate in the Mimoun

mechanism leads to the formation of aldehyde/ketone, but not epoxide.

Theoretical calculations make it possible to compare the reactivity of complexes that differ in the ligand

surroundings and the metal center under identical conditions, which is generally almost unattainable in experiment. Among

complexes of Group VI and VII elements of the periodic table, peroxo complexes of the elements from the third transition

row, W and Re, are most active in epoxidation. For structurally identical complexes from the Cr/Mo/W group, the activation

barriers of ethylene epoxidation decrease in a sequence Cr Mo W, and bisperoxo complexes of elements from this group

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are much more active than the corresponding monoperoxo complexes. Coordination of Lewis bases as ligands in the complex

increases the activation energy of the oxygen transfer due to a decrease in the electrophilicity of the peroxo group.

In TiIV systems, the hydroperoxo intermediates are characterized by much lower activation barriers of epoxidation

than the intermediates with a symmetric 2–O2 peroxo group. The Mo hydro- or alkylperoxo intermediates probably act as

epoxidizing intermediates in catalytic processes based on MoVI compounds.

Prospects for further studies are associated with the use of modern approaches to inclusion of solvent effects in

calculations, in particular, through combined use of quantum-mechanical and molecular mechanic (QM/MM) description and

detailed investigation of the mechanism of stereoselectivity control over oxygen transfer.

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